Two methods of power conversion are widely used. Linear regulation drops a supply current through a voltage equal to the desired difference between the supply voltage and the output voltage. Consequently, linear regulation wastes a great deal of power. For example, if a 5.0 volt input is to be dropped down to 3.3 volts the maximum power conversion efficiency that can be achieved is 66%. Efficiency is defined as the ratio of output power to input power. This means that 34% of the input power is wasted as heat dissipation.
Switching regulation is the other widely used method of power conversion. Switching regulators draw energy from an input supply in "packets" and temporarily store the energy in the magnetic field of a coil. The energy stored in the coil is then delivered to the load. The amount of power delivered to the load can be controlled by two methods. According to one method the amount of energy in each packet is held constant while the frequency at which the packets are drawn from the input supply is modulated. According to the second method the frequency at which the packets are drawn is held constant while the energy content of the packets is modulated in response to load requirements. Both methods generate noise pulses in the output because of the pulsed delivery of packets to the load. As a consequence, switching regulators typically use a low pass filter to remove the noise pulses from the output. That low pass filter may include a second coil to act as a choke. Switching voltage regulators cannot be fabricated as part of an integrated circuit, nor easily surface mounted within an integrated circuit package because they include coils.
The maximum theoretical power efficiency of a switching regulator is less than 100% because of the voltage drop across a coil-discharge diode within the regulator. The magnitude of the voltage drop can range between 0.5 volts to 0.7 volts. As a result, when stepping 5.0 volts down to 3.3 volts the maximum theoretical efficiency of a switching regulator is given by: ##EQU1## where V.sub.DROP is the voltage across the diode, and where .delta. is the duty cycle of coil energization, i.e., the fraction of the time the coil is connected to the supply. Under moderate Icad conditions, .delta. is approximately 0.5, which gives a theoretical maximum efficiently of approximately 90%. In practice, the maximum efficiency of switching regulators is less than 90% because of the non-ideality of switches and parasitic capacitance in the current path.